Polarization type stereoscopic image display devices enable an observer to respectively view the left-eye image and the right-eye image having different polarizations with the left eye and the right eye using polarizing eyeglasses, thereby allowing the stereoscopic images to be observed.
FIG. 1 shows a conventional stereoscopic image display device, and FIG. 2 shows a process of manufacturing the conventional stereoscopic image display device.
With reference to the drawings, the conventional stereoscopic image display device 10 comprises a left-eye image display 11, a right-eye image display 12, and a half mirror 13.
Also, the left-eye image display 11 and the right-eye image display 12 are installed at 90° to each other based on respective lower edges thereof, and output the left-eye image 11c and the right-eye image 12c, respectively.
Also, the displays 11, 12 respectively include polarizers 11d, 12d including panels 11a, 12a for producing the images 11c, 12c and polarizing films 11b, 12b which enable the images 11c, 12c to have predetermined polarizations 11d, 12d. 
That is, the images 11c, 12c are output each having their own predetermined polarizations 11d, 12d. 
Also, the half mirror 13 is installed between the displays 11, 12 forming an angle of 45° with each of the displays 11, 12, and the half mirror 13 functions to reflect the left-eye image 11c and transmit the right-eye image 12c, finally outputting the images 11c, 12c onto the front surface at the same time.
Furthermore, the polarizations 11d, 12d of the images 11c, 12c finally output by means of the half mirror 13 form 90° to each other.
Thus, when the observer is wearing the polarizing eyeglasses including polarizing filters 14a, 14b having a difference in polarizing angle of 90° to each other, he or she may view the left-eye image 11c only with the left eye and the right-eye image 12c only with the right eye, and thus the stereoscopic effect may be imparted to the user.
Meanwhile, the polarizations 11d, 12d of the images 11c, 12c may vary depending on the kind of the displays 11, 12, and the displays 11, 12 are currently manufactured so as to output the images having polarizations of 0°, 90° or 45°.
Also, the displays 11, 12 are manufactured according to different standards depending on the country, the manufacturer, or technique.
The half mirror 13 is configured such that the polarization passed through one side thereof is transmitted unchanged, and the polarization reflected from the other side thereof is converted and output.
That is, in the case where the polarization of 45° is reflected by means of the half mirror 13, the polarization of 45° is converted into a polarization of 135°.
However, in the case of the polarization of 0° or 90°, even when it is reflected and converted by means of the half mirror 13, it is mostly maintained at 0° or 90°.
This principle is to be understood that when the polarization of 0°, 90° or 45° is linearly symmetrically converted based on an imaginary vertical or horizontal line, the symmetrically converted polarizations are converted into 0°, 90° or 135°, respectively.
That is, in the case where a stereoscopic image display device is manufactured using two displays 11, 12′ for outputting images having polarizations of 0°, the polarizations of the images 11c, 12c reflected or transmitted by means of the half mirror 13 are 0° identical to each other, and thus the image is not transmitted through any one polarizing filter of the polarizing eyeglasses which the user is wearing. Ultimately, the stereoscopic effect is not imparted to the user.
Hence, the case where the stereoscopic image display device is manufactured using displays having the polarizations of 0° or 90° is problematic because the polarization of any one image should be rotated by 90° and then output.
Conventionally, the polarization 12d′ of any one image 12c is rotated by 90° in such a manner that the panel 12a and the polarizing film 12b of any one display 12′ among the displays 11, 12 are separated from each other, after which the polarizing film 12b′ is rotated by 90° and then re-attached to the panel 12a. 
However, such a conventional method undesirably degrades the color or view angle of the display, and there are many technical and temporal problems that follow from the separation and re-attaching of the polarizing film 12b′. 
Meanwhile, in the case where the stereoscopic image display device is manufactured using two displays (not shown) for outputting images having polarizations of 45% the polarization of the image output from any one display is converted by means of the half mirror 13 and thus has the polarization of 135°, thereby eliminating the need to separate and then re-attach the polarizing film 12′.
However, in the case where the stereoscopic image display device for simultaneously outputting the polarization of 45° and the polarization of 135° is installed at the same place along with a planar image display for outputting a planar image having the polarization of 45°, when a user is wearing the polarizing eyeglasses including polarizing filters having polarizing angles of 45° and 135°, the stereoscopic image output from the stereoscopic image display may be observed, but the planar image output from the planar image display may pass through only a single one polarizing filter of the polarizing eyeglasses and thus the user may undesirably observe the planar image only with either the left or the right eye.
FIGS. 7 and 8 show conventional stereoscopic image display systems.
With reference to the drawings, the conventional stereoscopic image display system comprises a stereoscopic image display device 10, 10a and a planar image display device 30, 30a. 
The description of the stereoscopic image display devices 10, 10a is substantially identical to that of the stereoscopic image display device as shown in FIGS. 1 and 2, and is thus omitted, and the same reference numerals are used.
The planar image display device 30, 30a outputs a planar image, in particular, a planar image having the polarization at the same angle as the polarization of the left-eye image 11c or the right-eye image 12c output from the stereoscopic image display device 10, 10a. 
That is, in the case where the stereoscopic image display device 10, 10a includes a stereoscopic image display device 10 for outputting the left-eye image and the right-eye image having polarizations 11d, 12d at 0° and 90°, respectively, the planar image display device 30, 30a includes a planar image display device 30 for outputting a planar image having a polarization 31 at 0° or 90°.
Also, in the case where the stereoscopic image display device 10, 10a includes a stereoscopic image display device 10a for outputting the left-eye image and the right-eye image having polarizations 11dd, 12dd at 45° and 135°, the planar image display device 30 may include a planar image display device 30a for outputting a planar image having a polarization 31a at 45° or 135°.
The reason is that the display is manufactured according to different standards adapted for different countries, manufacturers or techniques as mentioned above.
That is, in the case where the user who is wearing the polarizing eyeglasses 14, 14a observes the images output from the stereoscopic image display device 10, 10a and then views the planar image output from the planar image display device 30, 30a, there is the problem of the planar image not being able to pass through any one polarizing filter 14b, 14bb of the polarizing eyeglasses 14, 14a. 